1-MV Vacuum Insulation for the ITER Neutral Beam Injectors

The ITER is an international project which aims to develop an experimental reactor as a step to realize fusion energy. To inject 33 MW of 1 MeV D⁰ neutral beams for heating and current drive in ITER plasmas, D^- ions are accelerated to 1 MeV. The electrostatic accelerator consists of five acceleration stages, and each acceleration gap has to withstand 200 kV. Gaps between the accelerator and the vacuum vessel, which is at ground potential, must sustain voltages up to 1 MV, and high-voltage vacuum insulation is a key issue. A minimum gap length of >; 900 mm was selected for the l-MV insulation distance. Development of a high-voltage bushing (HVB) which is a bulkhead and a feedthrough between the gas insulated HV transmission line and the beam source in vacuum is ongoing. The HVB consists of a stack of five large bore ceramic rings (1.56 m diameter), each 0.29 m in height. The five-stage insulation concept was applied to both the accelerator and the HVB for better voltage holding with multiple short gaps compared to fewer longer gaps. R&D on a single-stage HVB ceramic ring with the associated electrostatic screens was carried out, and voltage holding of -203 kV dc for 5 h was confirmed.